human history
The history of population differentiations using genetic data was initially inferred from phylogenetic trees and from multivariate statistical methods such as principal components (of which multidimensional scaling is a derivative) that use allele frequencies.
Population trees are especially useful for reconstructing history if population differences can be assumed to result from fissions that occur randomly in time, with a constant rate of neutral evolution in each population between fissions.
This is likely to be roughly true for data on several autosomal genes from large populations that are geographically and genetically distant, which shows nine such groups from around the world.
Completely different types of DNA variation provide the same basic conclusion regarding the relationships between these populations.
Violation of the above assumptions, such as the presence of migration or selection, affects the interpretation of population trees.
However, when migration between geographic neighbors is frequent, principal components displayed in two dimensions reflect the geographical distribution of populations. Under the simple evolutionary model described above, trees and principal components give similar results.
For populations that are geographically close, genetic and geographic distances are often highly correlated, with an asymptote for the genetic distance at about 1,000−2,600 miles on average (but higher for Asia and the world, which are not at equilibrium).
Recent statistical developments in detecting clustering among populations based on highly polymorphic autosomal markers have been valuable for analyzing very large population genetic data sets.
It is important that this completely different approach produces the same primary continental clusters as the earlier methods. In its application to data sets with numerous polymorphic loci, however, it does seem to be more sensitive in detecting and assessing individual ancestry.
Early studies showed that genetic differences between populations are relatively small as compared with those within populations.
Subsequent analyses, including molecular polymorphisms of 14 populations representing all continents, confirmed that the within-population variance was about 85% of the total.
A recent analysis of 377 autosomal microsatellite markers12 in 1,065 individuals from 52 worldwide populations found that only 5−7% of the variation was between populations. It is the remaining 5−15%-the between-population component-that can be used to reconstruct the evolutionary history of populations.
Exemples
beta-globin gene studies
- The most recent common ancestor of the beta-globin gene tree is a sequence found only in Africa and estimated to have arisen approximately 800,000 years ago. (9106523)
- There is no evidence for an exponential expansion out of a bottlenecked founding population, and an effective population size of approximately 10,000 has been maintained. (9106523)
- Modest differences in levels of beta-globin diversity between Africa and Asia are better explained by greater African effective population size than by greater time depth. There may have been a reduction of Asian effective population size in recent evolutionary history. (9106523)
- Asian ancestry is estimated to be older than 200,000 years, suggesting that the ancestral hominid population at this time was widely dispersed across Africa and Asia. (9106523)
- Patterns of beta-globin diversity suggest extensive worldwide late Pleistocene gene flow and are not easily reconciled with a unidirectional migration out of Africa 100,000 years ago and total replacement of archaic populations in Asia. (9106523)
References
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Lahr MM, Foley RA. Towards a theory of modern human origins: geography, demography, and diversity in recent human evolution. Am J Phys Anthropol. 1998;Suppl 27:137-76. PMID: 9881525
Cavalli-Sforza LL. The DNA revolution in population genetics. Trends Genet. 1998 Feb;14(2):60-5. PMID: 9520599
Harding RM, Fullerton SM, Griffiths RC, Bond J, Cox MJ, Schneider JA, Moulin DS, Clegg JB. Archaic African and Asian lineages in the genetic ancestry of modern humans. Am J Hum Genet. 1997 Apr;60(4):772-89. PMID: 9106523
Cavalli-Sforza LL. Genes, peoples and languages. Sci Am. 1991 Nov;265(5):104-10. PMID: 1785035